Battery Safety has been discussed at length in the literature. Recently, the use of mathematical models to describe failure mechanisms and propagation modes within battery modules and packs has become more prevalent. Detailed models that represent thermal events from ohmic heating [1] to component-wise reaction heats, [2] gradual degradation of cell components, [3] plating of lithium leading to internal short circuits,[4] failure of welded joints,[5] venting of the cell via the crimp or the can walls,[6] mechanical deformation have been developed.[7]

This presentation will highlight the utility of physics-based models to discuss a few practical questions posed during the design process. We begin by attempting to present an electrochemical definition of a short circuit that is consistent across different failure modes. This section of the presentation will explore in detail the origin of failure within a cell. With our mathematical derivation, we identify parameters that govern the evolution of failure under a few key scenario, including an internal-short and mechanical crush. Next, we discuss different experimental approaches to measure these parameters and issues associated with the different measurement techniques.

The second part of the presentation will cover modeling tools for propagation of failure across a module. We will discuss the influence of different trigger mechanisms and compare the amounts of heat released across the different modes. The set of case-studies presented include gradual degradation of the interface, to ultra-high strain rate deformation scenario. We will present a methodology to distinguish between gradual deformation across prolonged periods of time and severe degradation across fewer use-cycles. These results are particularly relevant in designing duty cycles for different applications and to monitor how safety evolves as a function of the aging profile of the battery.

Finally we conclude with a generic map (Fig. 1) of the heat evolution versus dissipation rates that summarizes the various case-studies discussed in the presentation. This approach enables us to compare different cell designs for a given module requirement, determine packaging and/or spacing requirements as well as design of cooling loads that help mitigate the risk of failure propagation for a given design.

References:

1. J. Marcicki et al., J. Electrochem. Soc. 2017 164(1), A6440-A6448 doi: 10.1149/2.0661701jes

2. G-H. Kim, A. Pesaran and R. Spotnitz, J. Power Sources, 2017 170(2), p. 476-489.

3. R. Spotnitz, J. Power Sources, 2003, 113(1), p. 72-80.

4. P. Barai, K. Higa, V. Srinivasan, Phys. Chem. Chem. Phys., 2017,19, 20493-20505 10.1039/C7CP03304D

5. W. Cai, B. Kang, and S.J. Hu, (2017), Ultrasonic Welding of Lithium-Ion Batteries, ASME Press, ISBN: 0791861252

6. E. Darcy, J. Darst, W. Walker, S. Rickman, Cell Thermal Runaway Calorimetery, Presented at the AABC in San Francisco, CA (2017).

7. C. Zhang, J. Wu, L. Cao, Z. Wu and S. Santhanagopalan, J. Power Sources, 2017, 357, 126-137.